Selenium dioxide purification

References to recent examples may be found in the reviews of Owyang and Walker. Selenium dioxide has been superseded in most instances by DDQ, which generally gives higher yields and fewer purification problems. 

Selenium dioxide was obtained from Alfa Products, Morton/Thiokol Inc. and used without purification. 

Purification of the Organoselenium Compounds. After the oxidation of 2-butene with selenium dioxide was completed, the acetic acid solvent and the volatile reaction products were distilled at reduced pressure (10 mm. HgA). The residue, a yellow oil, was purified by adsorption chromatography in a column packed with silica gel. n-Hexane and ethyl ether were used as eluents. The same procedure was applied to the fractionation and purification of the organoselenium compounds obtained from the oxidation of bis(l-methyl-2-acetoxypropyl) selenide with peracetic acid. 

Purification may also be effected by oxidation to selenious acid, e.g. by heating with dilute nitric acid. On evaporation the solid selenium dioxide may be obtained, and this can be purified by repeated sublimation in a current of dust-free dry air.3 It may then be redissolved in water, the solution acidified with hydrochloric acid, and the selenium precipitated by passing in sulphur dioxide.4 For further purification the element can be sublimed in a current of carbon dioxide, and after heating for some time at 100° C. to convert it into the crystalline condition, it may be heated with carbon disulphide to extract any traces of residual sulphur. 

Very stable colloidal solutions of selenium may be prepared by the regulated action of concentrated hydrazine hydrate solution on selenium dioxide or grey crystalline selenium and subsequent dilution of the solutions with water and purification by dialysis. According to the degree of dispersion the colour of the solutions varies from intense yellow to blood red. These sols are completely irreversible. The dilute solutions are stable at the boiling-point, but are readily coagulated by barium sulphate. Sodium and potassium carbonates appear to increase the stability of the system. 

Selenium dioxide, unlike sulfur dioxide, is not readily prepared by direct combination of the elements. The best methods depend upon combustion of selenium in the presence of nitrogen dioxide1 or oxidation of selenium by nitric acid.2 The first method includes purification by sublimation the second gives a pure product only if the selenium is pure or if the product is sublimed. 

Selenium dioxide, by oxidation of selenium by nitric acid, 119 purification, 120 Selenium hexafluoride, 121 Silica gel, platinized, use in preparation of hydrogen bromide, 152 Silicobromoform, 38 analysis of, 41 Silicoethane, hexachlor, 42 Silicoformic anhydride, 42 Silicomolybdic acid, 127 analysis, 128... 

The chief advantages of the contact process are the high purity of the product and the fact that the product is a concentrated acid. Disadvantages are the high cost of the catalysts and the fact that if sulfides are used as raw materials, costly purification of the sulfur dioxide is necessary, because impurities such as arsenic trioxide and selenium dioxide poison the catalyst (i.e., render the catalyst inactive). Platinum catalysts are particularly sensitive to these impurities, while vanadium catalysts are claimed to be free from this disadvantage. 

Extrusion of sulfur dioxide, by warming in ethanol in the presence of sodium hydrogen carbonate, is followed by hydroxylation at C-1 with sub-stoichiometric amounts of selenium dioxide and N-methylmorpholine JV-oxide as the oxidant. The stereoselectivity of the oxidation lies at a ratio of 7 1 in favour of the (S)-diastereomer. After sUylation with t-butyldimethylsUyl chloride and chromatographic purification, the desired diastereomer is obtained in a yield of 41 %. 

Oxidation of 2-methoxycarbonylcyclopentan-2-one with selenium dioxide in refluxing dioxan gives (323) in 45% yield.The latter is thought to be the first simple cyclopentenone containing an electron-withdrawing substituent in position 2, a situation which probably enhances the readiness to polymerization exhibited by cyclopentenone. Indeed attempts at purification of (323) result in rapid polymerization. However, (323) may be trapped by formation of Diels-Alder adducts, in particular the adduct with cyclopentadiene on pyrolysis at 438 °C gave a pure sample of (323) which was trapped at —10 °C. 

Different types of methods for purification have been applied. Brauner in 1889, by means of aqua regia, converted crude tellurium into telluric chloride, the aqueous solution of which on precipitation by sulphur dioxide deposited the element together with selenium and traces of copper and lead as the only impurities. The precipitate was dissolved in fused potassium cyanide in an atmosphere of hydrogen and the tellurium separated from the aqueous extract by treatment with a current of air. The element was then distilled in a current of hydrogen, 1 an alternative is distillation in a vacuum.5 Another method of treatment for the telluric chloride solution is to precipitate in three fractions using sulphur dioxide, when it is found that the middle fraction is of a high degree of purity.6... 

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